The present invention relates to an improved process for producing squaric acid.
Squaric acid (quadratic acid; 1,2-dihydroxy-3,4-cyclobutenedione) is a valuable intermediate product for the manufacture of stabilizers, dyes, bactericides, and fungicides. A number of processes for its preparation are known, as described in detail in U.S. Pat. No. 4,097,530. According to the method disclosed in the aforementioned patent, squaric acid is prepared by reaction of hexachlorocyclobutene with 70-96% by weight sulfuric acid at temperatures of between 80.degree. and 150.degree. C. The best results are achieved by reacting hexachlorocyclobutene with 90% by weight sulfuric acid at 120.degree. C. In this process, 4-10 hours are required to react hexachlorocyclobutene to form squaric acid, and, in general, another 3 hours are needed to drive the reaction to completion and to evaporate the proportion of the resultant hydrogen chloride that has remained dissolved in the reaction mixture.
By raising the reaction temperature from 120.degree. C. to, for example, 140.degree. C., the required reaction period can be reduced, as expected, but in that case, tars are generated which discolor the squaric acid (Example 7). The reference discloses that, if the sulfuric acid concentration deviates markedly from the optimum value of 90% by weight in the downward as well as upward directions, so that it is outside the range 70-96%, the reaction is unsuccessful. Only small amounts of squaric acid are formed below a sulfuric acid concentration of 70% by weight, even with the use of higher temperatures and pressures. Above a sulfuric acid concentration of 96% weight, there is almost no hydrolysis because of the low amount of water present (column 2, lines 8-16).
The above-mentioned patent describes the possibility of adding water (Example 5) or dilute sulfuric acid (Example 3 of the reference) during the reaction, or initially using adequate amounts of 90% strength sulfuric acid, in order to maintain an optimum water concentration. Also,in the examples of the reference, the sulfuric acid concentration during the entire reaction is maintained well below 96% by weight in all cases.
Thus, the teaching of this reference is to start with a sulfuric acid concentration of not more than 96% and to avoid permitting the acid concentration to rise above 96% at any time during the reaction. Water is added to keep the acid concentration below 96%. There is no recognition that it might be advantageous to start the reaction at a sulfuric acid concentration above 96% and to maintain the acid at a concentration above 96% during the entire course of the reaction. Rather, this is directly contrary to the teaching of this reference and to the entire thrust of its disclosure.
Although the above-described process also permits use of mixtures of hexachlorocyclobutene with hexachlorobutadiene, Example 4 of the reference demonstrates that reaction of a mixture of 50% each of hexachlorocyclobutene and hexachlorobutadiene results in a yield that is reduced with respect to the optimum by 20%, at 120.degree. C., and the product is only obtained after a reaction time of 15 hours. Such results are unacceptable for industrial processes. Furthermore, the reaction product becomes colored after such a long reaction period. Mixtures wherein the hexachlorocyclobutene proportion is even lower are hardly usable at all.
On the other hand, great interest exists, for economic and industrial reasons, in being able to utilize mixtures having as low a proportion of hexachlorocyclobutene as possible. As is known, such mixtures can be produced more cheaply because of lower distillation costs (see German Patent 2,618,557). Solutions of such a low percentage have the further advantage in handling that the point of crystallization of the cyclic isomer is so low as to prevent it from precipitating from the solution at room temperature.
German Patent 2,824,558 describes another process for the preparation of squaric acid wherein hexachlorocyclobutene or its mixture with hexachlorobutadiene is initially heated with oleum (fuming sulfuric acid, i.e., containing at least 10 mole % SO.sub.3 in 100% H.sub.2 SO.sub.4) to 60.degree.-120.degree. C. and subsequently, in a second process step, in the reaction mixture, the organic intermediate is hydrolyzed to squaric acid and the thus-formed chlorosulfonic acid is hydrolyzed to sulfuric acid.
Apart from the fact that industrial use of oleum is best avoided, for financial reasons and on account of its aggressive and toxic effects, this process has additional drawbacks. Special precautions are necessary to hydrolyze the chlorosulfonic acid formed during the reaction. An unexpected and troublesome occurrence during the reaction is the formation of products that are extremely lachrymatory. Under these circumstances, realization of the process under industrial conditions is fraught with considerable technical and toxicological difficulties.
German Patent 2,824,558 does not contain any suggestion of operating with oleum having a low SO.sub.3 concentration, (i.e., less than 10 mole % SO.sub.3, because in such a case the quantity of sulfuric acid would have to be significantly increased.
A need therefore continues to exist for a process for the production of squaric acid which avoids these disadvantages.